The software does not limit the number of concurrent zones. Generally when a zone is running, it draws somewhere around 200 to 300mA current. So you can multiply that by the number of zones you want to run simultaneously to get the minimum current rating of the power supply. I think if you plan to run 3 zones at the same time, the 1 amp power supply that comes with OpenSprinkler DC is sufficient. Beyond that you may want to upgrade to a 1.5 amp or 2 amp power supply.